Light guide body having high luminous intensity and high transparency

ABSTRACT

The present invention relates to light guide bodies having improved luminous intensity and transparency, to a method for their production and to their use.

FIELD OF THE INVENTION

The present invention relates to light guide bodies having improvedluminous intensity and transparency, to a method for their productionand to their use.

BACKGROUND OF THE INVENTION

Light guide bodies which are based on the principle of embeddingscattering particles in a transparent thermoplastic matrix are known.

For instance, EP 656 548 discloses light guide bodies which use polymerparticles as scattering particles. More than 98% of the polymerparticles have a size of at least 7 micrometres, and from 0.01 wt % (100wt·ppm) to 1 wt % (10,000 wt·ppm) of them are added to a matrix plastic.These light guide bodies have the disadvantage that their weatheringresistance is insufficient and they already exhibit cloudiness—expressedby the “haze” value—at a thickness of 1 mm, in particular above 2 mm.

EP 1453900 describes light guide bodies which contain barium sulphatewith an average particle size of 0.3-20 micrometres as scatteringparticles in a concentration of 0.001 wt % (10 wt·ppm)-0.08 wt % (800wt·ppm). These light guide bodies also have the disadvantage that theyalready exhibit cloudiness at a thickness of 1 mm, in particular above 2mm.

The above-described light guide bodies of the prior art also have thedisadvantage that a low luminance is achieved with observationperpendicular to the light emitting surface. Only at large angles,measured with respect to the surface normal of the light emittingsurface (cf. FIG. 1), does the luminance increase significantly. Thiseffect is represented in FIG. 2 for various scattering particles.

Besides light guide bodies comprising scattering particles, there arealso light guide bodies which are produced by mechanical introduction ofdefects on the surface or by printing on the surface. Production ofthese light guide bodies, however, involves much greater productionoutlay. Furthermore, printing in particular causes poor transparency andtherefore restricts the possibilities of the use of these light guidebodies. Light guide bodies having embedded scattering particles,conversely, can be produced economically by extrusion, injectionmoulding or by casting methods without post-processing. Examples of suchprinted light guide bodies may be found in JP2004351649, WO 2007/058060,WO 2009/137053, US 2005/272879.

SUMMARY OF THE INVENTION

In view of the prior art discussed above, it was therefore an object ofthe present invention to provide light guide bodies and a method fortheir production, which do not have the disadvantages of the light guidebodies of the prior art or have them only to a reduced extent.

It was a particular object to provide light guide bodies which exhibitlittle cloudiness in through-view, i.e. in the direction of the surfacenormals, and can therefore be used in application fields which areincompatible with light guide bodies of the prior art because of opticalrequirements.

It was another particular object to provide light guide bodies whichhave a high luminance at low angles, measured with respect to thesurface normal of the light emitting surface.

It was also an object of the invention to provide light guide bodieswhich can be produced in a particularly simple way, for example byextrusion, injection moulding or by casting methods.

It was furthermore an object of the present invention to provide lightguide bodies which can be produced economically.

Lastly, it was also an object of the present invention to provide lightguide bodies which can be adapted easily to requirements in respect ofsize and shape.

Other not explicitly mentioned objects may be inferred from the overallcontext of the following description, examples and claims.

These objects are achieved by the light guide bodies described in claim1. Expedient variants of the light guide bodies according to theinvention are placed under protection by the dependent claims referringto claim 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a light guide body.

FIG. 2 depicts the effect of luminance as a function of angle to thesurface in a light guide body for various scattering particles.

FIG. 3 depicts a light guide body illustrating the entrance of lightperpendicular to the light emitting surface.

FIG. 4 depicts the effect of luminance as a function of angle to thesurface in a light guide body for barium sulfate and titanium dioxideparticles.

FIG. 5 depicts the effect of luminance as a function of angle to thesurface in a light guide body for barium sulfate and titanium dioxideparticles.

FIG. 6 depicts scattering efficency as a function of titanium dioxideparticle diameter at 400 nm, 500 nm and 650 nm in a light guide body.

FIG. 7 depicts another embodiment of a light guide body.

DETAILED DESCRIPTION OF THE INVENTION

In respect of the method for producing the light guide bodies accordingto the invention, Claims 10 to 12 and the following description providea solution to the underlying object.

Before the present invention is described in detail, some importantterms will be defined as follows (cf. FIGS. 1 and 3):

The term light emitting surface denotes a surface of the light guidebody which is intended to emit light. The light entry surface is in turncapable of receiving light into the light guide body, so that the lightguide body can distribute the introduced light substantially uniformlyover the light emitting surface(s).

The light guide bodies according to the invention preferably have atleast a thickness of 1 mm. The thickness particularly preferably lies inthe range of from 2 to 25 mm and more particularly preferably is from 3to 20 mm. The light guide body may, as indicated in FIG. 1, also beassigned further layers, for example mirror or reflection layers.

Surprisingly, it has been found that the disadvantages of conventionallight guide bodies can be overcome by embedding titanium dioxidescattering particles with an average particle size of from 150 to 500nm. For example, the inventors have found that light guide bodiesaccording to the invention, containing titanium dioxide particles withthis special average particle size, are significantly more transparentin comparison with light guide bodies having for example BaSO₄scattering particles, despite a significantly higher light intensity ata small angle with respect to the surface normal of the light emittingsurface. The light guide bodies according to the invention preferablyhave at thickness—measured at the thinnest position—of at least 1 mm,particularly preferably from 2 to 25 mm and more particularly preferablyfrom 3 to 20 mm. The light guide bodies according to the invention arealso distinguished by very low cloudiness, measured as haze according toASTM D1003, which is less than 4%, preferably less than 2%, particularlypreferably from 0.2 to 2%. In contrast to the light guide bodies of theprior art, the light guide bodies according to the invention thereforeappear substantially transparent to the human eye and can also be usedin application fields which have great requirements for opticalproperties. The light guide bodies according to the invention areparticularly preferably distinguished in that they have no printingperceptible to the human eye at an observation distance of 1 m, and moreparticularly preferably no printing at all, in particular with dyes, onthe surface or in another layer of the shaped body. In another preferredembodiment, the light guide bodies according to the invention also haveno other optical defects on the surface or in a layer. In contrast tothe light guide bodies of the prior art, it has therefore been possiblefor the first time to achieve very uniform light output over the entirelight guide body in combination with a very high transparency thereof,without having to carry out the additional working step of applyingprinting or defects.

The possibility of being able to obviate printing and defects moreoverallows the light guide bodies according to the invention to beconfigured also as freeform bodies with any desired spatialconfiguration. In contrast to plate-shaped light guide bodies of theprior art, in which the pattern of the defects or printing can becalculated simply, this is not possible with complicated configurations.In a particularly preferred embodiment, the light guide bodies accordingto the invention are therefore freeform bodies with high transparencyand uniform light output, so that a hitherto impossible configurationalfreedom is achieved.

Transparent light guide bodies offer many practical as well asdecorative applications. For instance, it is possible to configureglazing which ensures a clear, distortion-free and uninterrupted viewthrough when the light source is switched off. When the light sourcesare switched on, the glazing acts as a uniform flat light source whichcan be used for general or ambient lighting.

Light guide bodies, which contain titanium dioxide scattering particlesthat have an average particle size of from 150 to 500 nm, emit lightvery much more uniformly over all angles, measured with respect to thesurface normal of the light guide bodies, than light guide bodiesaccording to the prior art, for example of EP 1453900. This isrepresented in FIG. 4 and FIG. 5. The luminance of the light guidebodies according to the invention is therefore significantly higherunder perpendicular observation then when conventional scatteringparticles are used.

The use of titanium dioxide particles as scattering particles in lightguide bodies is known per se. JP2002148443 describes, for example, lightguide bodies which contain titanium dioxide with a particle size of lessthan 100 nm. The inventors have, however, discovered that titaniumdioxide has its highest scattering efficiency in the scattering bodiesaccording to the invention, almost independently of the wavelength ofthe light, with a particle size of between 150 and 500 nm. This isillustrated in FIG. 6 with the aid of a Mie scattering simulation oftitanium dioxide (rutile modification) in polymethyl methacrylate. Below150 nm titanium dioxide particles, i.e. those according to JP2002148443,scatter light increasingly wavelength-selectively. That is to say, bluelight (wavelength 450 nm) is scattered significantly more than greenlight (550 nm) and even more so than red light (650 nm). The effect ofthis is that white light fed into the light guide at the light entrysurfaces is demixed on its way through the light guide. At a shortdistance from the light entry surfaces, the light emerging from thelight emitting surfaces is therefore blueish, and at a large distancefrom the light entry surfaces it is increasingly yellowish to reddish.This leads to a perturbing wavelength dispersion, that is to say thelight fed in emerges again with a colour distortion at the lightemitting surfaces. This problem is resolved by the present invention.The titanium dioxide particles used according to the invention thereforepreferably have an average particle size of from 150 to 500 nm,particularly preferably from 160 to 450 nm, more particularly preferablyfrom 170 to 450 nm, especially preferably from 200 to 400 nm, and moreespecially preferably from 220 to 400 nm. Titanium dioxide particleswhich have the aforementioned properties are commercially available (forexample KRONOS 2220 from Kronos Titan or HOMBITAN R 610 K fromSachtleben Chemie). Various methods for their production are known.

The proportion of titanium dioxide particles in the light guide body isfrom 0.1-100 wt·ppm, preferably from 0.1 to 50 wt·ppm and particularlypreferably from 0.1 to 10 wt·ppm, in each case expressed in terms of theweight of the light guide body. Despite the improved light intensity, itis therefore possible to achieve a significant reduction of theproportion of scattering particles, and therefore a cost advantage. Thereduced proportion also has a positive effect on the transparency, sothat even light guide bodies with a thickness of at least 1 mm,preferably at least 2 mm, can be produced with high transparency andscattering intensity. Furthermore, a high ratio of the light emittingsurface to the light introduction surface of at least 4, preferably atleast 10, particularly preferably at least 20, and more particularlypreferably at least 80 can be achieved. This means that the light guidebody of the present invention differs to a great extent from knowncovers for lighting bodies. Specifically, known covers are distinguishedin that the light introduction surface is formed parallel to the lightemitting surface, so that both surfaces have approximately the samesize.

Light guide bodies according to the invention preferably have a lengthin the range of from 25 mm to 3000 mm, preferably from 50 to 2000 mm andparticularly preferably from 200 to 2000 mm. The width preferably liesin the range of from 25 to 3000 mm, preferably from 50 to 2000 mm andparticularly preferably from 200 to 2000 mm.

Cubic configurations are possible, but also configurations taperingtoward one side, which have a wedge shape. In the case of a wedge shape,light is preferably input only through one light entry surface.

The light preferably enters the light guide body perpendicularly to thelight emitting surface, i.e. the light entry surface is perpendicular tothe light emitting surface. Examples of this embodiment can be found inFIGS. 1 and 3. In an alternative embodiment, however, the light guidebodies according to the invention also allow the light entry surface notto be perpendicular to the light emitting surface and a high lightintensity is nevertheless still achieved. This is possible, for example,when the light guide body is configured so that the entering light isguided into the light guide surface by corresponding refraction orreflection at special layers. An example of this is represented in FIG.7. The light guide bodies according to the invention therefore allowsignificantly improved configurational latitude for the user ordesigner.

As a light source for illuminating the light entry surfaces of the lightguide bodies according to the invention, fluorescent lamps,light-emitting diodes, incandescent lamps and halogen discharge lampsmay be used. Depending on the arrangement of the light sources, thelight may be shone in through all four edges. This may be necessary inparticular for very large light guide bodies. In the case of smallerlight guide bodies, it is generally sufficient to illuminate one or twoedges of the light guide body. For better exploitation of the lightenergy used, those edges which are not provided with a light source maybe configured to be reflective. This configuration may for example becarried out using reflective adhesive tapes. Furthermore, a reflectivecoating may be applied onto these edges.

JP 7020459 describes light guide bodies which contain titanium dioxidein the anatase modification as scattering particles. The inventors havefound that titanium dioxide having a high proportion of anatase, incombination with moisture and oxygen, can lead to the accelerateddestruction of the light guide body. In a preferred embodiment, thelight guide bodies according to the invention therefore comprisetitanium dioxide particles having a proportion of the rutilemodification equal to at least 50 wt %, preferably at least 60 wt %,particularly preferably at least 70 wt % and more particularly at least90 wt %.

In order to produce the light guide bodies according to the invention,that is to say as transparent thermoplastic or thermoelastic plasticsfor embedding the titanium dioxide particles, transparentpolycarbonates, cyclo-olefin copolymers, polystyrenes, polyesters andparticularly (meth)acrylate polymers are suitable.

These (meth)acrylate polymers are generally obtained by radicalpolymerization of monomer mixtures. The monomer mixtures moreparticularly preferably contain methyl methacrylate, in particular atleast 40 wt % of methyl methacrylate, preferably at least 60 wt % andparticularly preferably at least 80 wt %, expressed in terms of theweight of the monomer mixture.

Besides this, these monomer mixtures may contain further (meth)acrylateswhich are copolymerizable with methyl methacrylate. The expression(meth)acrylates covers methacrylates and acrylates, as well as mixturesof the two. These monomers are widely known. They include inter alia(meth)acrylates which are derived from saturated alcohols, for examplemethyl acrylate, ethyl(meth)acrylate, propyl(meth)acrylate,n-butyl(meth)acrylate, tert.-butyl(meth)acrylate, pentyl(meth)acrylateand 2-ethylhexyl(meth)acrylate; (meth)acrylates, which are derived fromunsaturated alcohols, for example oleyl(meth)acrylate,2-propynyl(meth)acrylate, allyl(meth)acrylate, vinyl(meth)acrylate;aryl(meth)acrylates such as benzyl(meth)acrylate orphenyl(meth)acrylate, in which case the aryl radicals may respectivelybe unsubstituted or substituted up to 4 times;cycloalkyl(meth)acrylates, such as 3-vinylcyclohexyl(meth)acrylate,bornyl(meth)acrylate; hydroxylalkyl(meth)acrylates such as3-hydroxypropyl(meth)acrylate, 3,4-dihydroxybutyl(meth)acrylate,2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate; glycoldi(meth)acrylates such as 1,4-butanediol(meth)acrylate, (meth)acrylatesof ether alcohols, such as tetrahydrofurfuryl(meth)acrylate,vinyloxyethoxyethyl(meth)acrylate; amides and nitriles of (meth)acrylicacid, such as N-(3-dimethylaminopropyl)(meth)acrylamide,N-(diethylphosphono)(meth)acrylamide,1-methacryloylamido-2-methyl-2-propanol; methacrylates containingsulfur, such as ethylsulphinylethyl(meth)acrylate,4-thiocyanatobutyl(meth)acrylate, ethylsulphonylethyl(meth)acrylate,thiocyanatomethyl(meth)acrylate, methylsulphinylmethyl(meth)acrylate,bis((meth)acryloyloxyethyl)sulphide; polyvalent (meth)acrylates such astrimethyloylpropane tri(meth)acrylate.

Besides the (meth)acrylates mentioned above, the compositions to bepolymerized may also comprise further unsaturated monomers which arecopolymerizable with methyl methacrylate and the aforementioned(meth)acrylates.

These include inter alia 1-alkenes such as hexene-1, heptene-1; branchedalkenes, for example vinylcyclohexane, 3,3-dimethyl-1-propene,3-methyl-1-diisobutylene, 4-methylpentene-1; acrylonitrile; vinyl esterssuch as vinyl acetate; styrene, substituted styrenes having an alkylsubstituent in the side chain, for example α-methylstyrene andα-ethylstyrene, substituted styrenes having an alkyl substituent on thering, such as vinyl toluene and p-methylstyrene, halogenated styrenes,for example monochlorostyrenes, dichlorostyrenes, tribromostyrenes andtetrabromostyrenes; heterocyclic vinyl compounds such as2-vinylpyridine, 3-vinylpyridine, 2-methyl-5-vinylpyridine,3-ethyl-4-vinylpyridine, 2,3-dimethyl-5-vinylpyridine, vinylpyrimidine,vinylpiperidine, 9-vinylcarbazole, 3-vinylcarbazole, 4-vinylcarbazole,1-vinylimidazole, 2-methyl-1-vinylimidazole, N-vinylpyrrolidone,2-vinylpyrrolidone, N-vinylpyrrolidine, 3-vinylpyrrolidine,N-vinylcaprolactam, N-vinylbutyrolactam, vinyloxolane, vinylfuran,vinylthiophene, vinylthiolane, vinylthiazoles and hydrogenatedvinylthiazoles, vinyloxazoles and hydrogenated vinyloxazoles; vinyl andisoprenyl ethers; maleic acid derivatives, for example maleic anhydride,methylmaleic anhydride, maleinimide, methylmaleimide; and dienes, forexample divinylbenzene.

In general, these comonomers are used in an amount of from 0 to 60 wt %,preferably 0 to 40 wt % and particularly preferably 0 to 20 wt %,expressed in terms of the weight of the monomers, in which case thecompounds may be used individually or as a mixture.

The polymerization is generally started using known radical initiators.The preferred initiators include inter alia the azo initiators widelyknown in the technical field, such as AIBN and1,1-azobiscyclohexanecarbonitrile, and peroxy compounds, such asmethylethylketone peroxide, acetylacetone peroxide, dilauryl peroxide,tert.-butylper-2-ethyl hexanoate, ketone peroxide, methylisobutylketoneperoxide, cyclohexanone peroxide, dibenzoyl peroxide,tert.-butylperoxybenzoate, tert.-butylperoxyisopropyl carbonate,2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane,tert.-butylperoxy-2-ethyl hexanoate, tert.-butylperoxy-3,5,5-trimethylhexanoate, dicumyl peroxide, 1,1-bis(tert.-butylperoxy)cyclohexane,1,1-bis(tert.-butylperoxy)-3,3,5-trimethylcyclohexane, cumylhydroperoxide, tert.-butyl hydroperoxide,bis(4-tert.-butylcyclohexyl)peroxydicarbonate, mixtures of two or moreof the aforementioned compounds with one another and mixtures of theaforementioned compounds with compounds not mentioned, which canlikewise form radicals.

These compounds are often used in an amount of from 0.01 to 10 wt %,preferably from 0.5 to 3 wt %, expressed in terms of the weight of themonomers.

Various poly(meth)acrylates may be used, which differ for example interms of molecular weight or monomer composition.

In order to produce the light guide bodies according to the invention,the monomer mixtures may be cast into the desired shape and thenpolymerized. Both discontinuous and continuous casting methods may beemployed as casting methods.

It is, however, also possible first to produce a moulding compound andform the light guide body therefrom.

Such particularly preferred moulding compounds are commerciallyavailable under the brand name PLEXIGLAS® from Röhm GmbH.

The weight average of the molecular weight Mw of the moulding compoundsmay vary in wide ranges, the molecular weight usually been adapted tothe application purpose and the way in which the moulding compound isprocessed. In general, however, it lies in the range of between 20,000and 1,000,000 g/mol, preferably from 50,000 to 500,000 g/mol andparticularly preferably from 80,000 to 300,000 g/mol, without this beingintended to form any restriction.

After the titanium dioxide particles have been added, light guide bodiescan be produced from these moulding compounds by conventionalthermoplastic shaping methods. These include in particular extrusion andinjection moulding.

In a particularly preferred embodiment, the light guide bodies accordingto the invention are produced by multicomponent, preferably 2-component,injection moulding, only one component (moulding compound) containingthe titanium dioxide particles and the other not containing anyscattering particles. In this way, it is possible to produce light guidebodies in which the component containing titanium dioxide represents aparticular pattern. In the switched-off state, that is to say withoutlight entry, the shaped body appears as a homogeneous transparent shapedbody, and the pattern cannot be seen. After switching on, however, thepattern can be seen since only the part of the shaped body whichcontains the titanium dioxide particles outputs the light.

In a preferred embodiment, the present invention therefore also relatesto light guide bodies which have light scattering means and a hazeaccording to Claim 1 only in particular regions. The remaining regionsare particularly preferably configured not to be light-scattering. Inthis way, for example, warning signs can be produced or also covers offor example vehicle cockpits, in which the warning symbols appearthree-dimensionally highlighted after switching on (“floating symbols”).

The freeform bodies according to the invention may preferably beproduced by injection moulding or by thermoforming previously producedlight guide plates. In particular, freeform bodies illuminating throughthe base of a thermoplastic may be produced. For example, a curvedilluminating display in a motor vehicle interior or contours of awashing machine may be produced using thermoformed plates or directly byinjection moulding.

By the obviation of printing, an additional process step and concomitantextra costs are avoided. In comparison with light guide bodies which areproduced by means of injection moulding in structured moulds, a mouldingcompound according to the invention, comprising scattering bodies, makesit possible to obviate expensive structuring of the injection mould, andit furthermore allows shorter cycle times since no microstructures haveto be formed precisely, as well as greater productivities. Furthermore,configuring microstructures in freeform surfaces is much more elaboratein terms of lighting technology.

As already mentioned, light guide bodies of the present invention mayalso be produced by casting methods. In this case, acrylic resinmixtures are preferably put into a mould and polymerized.

A preferred acrylic resin comprises, for example,

-   -   A) 0.1-100 wt·ppm of titanium dioxide particles having an        average diameter in the range of from 150 to 500 nm,    -   B) 40-99.999 wt % of methyl methacrylate,    -   C) 0-59.999 wt % of the above-defined comonomers,    -   D) 0-59.999 wt % of polymers soluble in (B) or (C),        the components A) to D) adding up to 100 wt %.

The acrylic resin furthermore comprises the initiators necessary for thepolymerization. The components A to D and the initiators correspond tothe compounds which are also used to produce suitable polymethylmethacrylate moulding compounds.

The moulding compounds and the acrylic resins may contain conventionaladditives of all types. These include inter alia antistatics,antioxidants, mould release agents, flameproofing agents, lubricants,dyes, flow enhancers, fillers, photostabilizers and organic phosphoruscompounds such as phosphites or phosphonates, pigments, weatheringprotection agents and plasticizers.

The amount of additives is, however, restricted by the applicationpurpose. For instance, the light guiding property of the polymethylmethacrylate light guide bodies must not be compromised too greatly byadditives.

The light guide bodies of the present invention may be used toilluminate LCD displays, information signs and advertising panels. Thelight guide bodies according to the invention are particularlypreferably employed as transparent glazing elements, which can be usedas flat lights by switching on the light sources fitted on the lightentry surfaces.

The light guide bodies according to the invention may furthermore beused to produce signs, display elements, advertising objects etc., whichrender particular symbols or signs visible only after switching on.

Measurement Methods:

Proportion of TiO₂ in the Light Guide Body

The light guide body is ashed. The mass of ash remaining is put inproportion to the mass of the light guide body. In this case, in thescope of the present invention, the entire mass of the ash is equated tothe TiO₂ content.

Average Particle Size of the TiO₂ Particles in the Light Guide Body

Images of the light guide body are recorded using a transmissionelectron microscope. The particle diameter of the titanium dioxideparticles embedded in the light guide body is determined by taking anaverage of the largest and smallest extent of the titanium dioxideparticle in question. The average particle size is determined from 50titanium dioxide particle diameters. The sample preparation of the lightguide bodies for the electron microscopy study is carried out accordingto conventional methods known to the person skilled in the art.

Proportion of the Rutile Modification of the TiO₂ Particles

The modification of the titanium dioxide particles is determined bymeans of Raman spectroscopy. The proportion of rutile particles inrelation to the particles measured overall is then determined. At least100 titanium dioxide particles are preferably analysed. In order toincrease the measurement accuracy, even more particles may be studied ifnecessary.

Luminance

The luminance is measured using a commercially available luminancemeasuring instrument (for example from LMT or Minolta).

The following examples serve to illustrate and further explain thepresent invention, but in no way restrict it.

Comparative Example 1

In a single-screw extruder, polymethyl methacrylate moulding compoundwas mixed with a master batch containing barium sulphate and melted. Theresulting melt contained 160 wt·ppm of barium sulphate, expressed interms of the weight of the polymethyl methacrylate, with an averageparticle size of 3 micrometres. The melt was removed from the extruderby means of a wide-slit nozzle and shaped into a polymethyl methacrylateplate in a smoothing mechanism. A rectangular light guide body with alength of 900 mm, a width of 500 mm and a thickness of 4 mm was cut fromthe polymethyl methacrylate plate. This light guide body was provided onall 4 mm thick edges (light entry surfaces) with light-emitting diodeswhich emit the light into the edges. A diffusely reflecting white sheetwas arranged on one surface of the light guide body (light emittingsurface 1). Centrally on the opposite surface of the light guide body(light emitting surface 2), the luminance was measured at various angleswith respect to the surface normal of the light emitting surface 2 ofthe light guide body.

A sample piece with a length of 100 mm, a width of 100 mm and athickness of 4 mm was cut from the same extruded light guide body, andthe cloudiness of the 4 mm thick sample piece was measured.

Example 1

A 4 mm thick plate was formed as in Comparative Example 1, although itcontains 1.2 wt·ppm of titanium dioxide with a particle size of 200 nminstead of barium sulphate. Luminance at various angles and thecloudiness were determined as in Example 1.

FIG. 4 shows that the luminance of the light guide body to whichtitanium dioxide is added is distributed more uniformly as a function ofthe angle with respect to the surface normal, compared with theluminance of a light guide body to which barium sulphate is added. Itcan furthermore be seen that the luminance in the direction of thesurface normal, i.e. at the angle 0°, is higher for the plate comprisingtitanium dioxide than for the plate comprising barium sulphate.

The cloudiness of the titanium dioxide light guide body is furthermoremuch less than the cloudiness of the light guide body to which bariumsulphate is added.

Example 2

To a polymerizable solution of polymethyl methacrylate in methylmethacrylate, 1.2 wt·ppm of titanium dioxide particles expressed interms of the weight of the solution with an average particle size of 200nm were added. A radical polymerization initiator, a UV absorber and aseparating agent were added to the solution. The solution was introducedinto a chamber consisting of two silicate glass sheets, which isprovided with a circumferential sealing strip, and prepolymerized at 60°C. then finally polymerized at 120° C. A light guide body with a lengthof 900 mm and a width of 500 mm was cut from the 4 mm thick polymethylmethacrylate plate obtained in this way. The light guide body wasprovided on all sides with light-emitting diodes which emit their lightinto all the 4 mm thick edges (light entry surfaces) of the light guidebody. A diffusely reflecting white sheet was arranged on one surface ofthe light guide body (light emitting surface 1). On the opposite surfaceof the plate (light emitting surface 2), the luminance was measured atvarious angles to the surface normal of the surface of the light guidebody.

A sample piece with a length of 100 mm, a width of 100 mm and athickness of 4 mm was cut from the same plate, and the cloudiness of the4 mm thick sample piece was measured as a haze value.

FIG. 5 shows that in the light guide body produced by radicalpolymerization using the casting method, the luminance of the lightguide body to which titanium dioxide is added is again uniform as afunction of the angle with respect to the surface normal. It canfurthermore be seen that, as in Example 1, the luminance of the lightguide body comprising titanium dioxide in the direction of the surfacenormal is higher than in the light guide body comprising bariumsulphate.

Table 1 compares the luminances of the extruded light guide bodycomprising 3 micrometre barium sulphate scattering particles fromComparative Example 1 with the extruded light guide body comprising 200nm titanium dioxide scattering particles from Example 1 and the cast(radical polymerized) light guide body comprising 200 nm titaniumdioxide scattering particles from Example 2.

Table 2 compares the cloudiness of the extruded light guide bodycomprising 3 micrometre barium sulphate scattering particles fromExample 1 with the extruded light guide body comprising 200 nm titaniumdioxide scattering particles from Example 2 and the cast(radical-polymerized) light guide body comprising 200 nm titaniumdioxide scattering particles from Example 3. At the same thickness, thelight guide bodies produced with titanium dioxide scattering particleshave significantly less cloudiness, measured as haze, and aretransparent when observed visually. TABLE 1 Luminance in surface normalScattering Concentration direction particles [wt. ppm] [cd/m²]Comparative Barium 160 580 Example 1 sulphate Example 1 Titanium 1.2 825dioxide Example 2 Titanium 1.2 783 dioxide

TABLE 2 Scattering Concentration particles [wt. ppm] Haze [%]Comparative Barium 160 21.07 Example 1 sulphate Example 1 Titanium 1.21.92 dioxide Example 2 Titanium 1.2 1.47 dioxide

Example 3 Freeform Body Exemplary Embodiment

A freeform body was produced from a light guide plate according toExample 1 by means of a thermoforming process. It had no printing on thesurface and nevertheless exhibited the appearance of a uniform light.

Example 4 Floating Symbol Exemplary Embodiment

A floating navigation arrow in a motor vehicle instrument cluster wasproduced by the following technique:

By two-component injection moulding, the navigation arrow is injectionmoulded using a light-scattering moulding compound according to theinvention. This arrow is embedded into a second glass-clear mouldingcompound component. This corresponds substantially to inlaying based oninjection moulding.

The semifinished product was then placed centrally in an instrumentcluster and illuminated from the side using LEDs. In the unilluminatedstate, the car driver sees through the fully transparent plate and doesnot perceive any contours. As a result of the illumination, the lightpasses through the glass-clear moulding compound as through a waveguide,and it is therefore not luminous. In the region of the navigation arrow,the light-scattering particles output the light and the contour of thenavigation arrow is lit in a floating fashion. This technique can beapplied to various symbol illuminations.

1. Light guide body having at least one light entry surface and at leastone light emitting surface, characterized in that the ratio of the lightemitting surface to the light entry surface is at least 4, in that ithas a haze, measured according to ASTM D1003, of less than 4%, and inthat titanium dioxide particles with an average particle size of 150-500nm are used as light scattering means in a concentration of 0.1-100wt·ppm, expressed in terms of the weight of the light guide body. 2.Light guide body according to claim 1, characterized in that at least50% of the titanium dioxide particles are present in the rutilemodification.
 3. Light guide body according to claim 1 or 2,characterized in that the ratio of the light emitting surface to thelight entry surface is at least
 20. 4. Light guide body according to oneor more of the preceding claims, characterized in that it consists ofthe at least one transparent thermoplastic or thermoelastic plasticand/or in that it has light scattering means and a haze according toclaim 1 only in particular regions and/or in that it has regions whichare not configured to be light-scattering.
 5. Light guide body accordingto one or more of the preceding claims, characterized in that titaniumdioxide particles are used as light scattering means in a concentrationof 0.1-50 wt·ppm, expressed in terms of the weight of the light guidebody, preferably 0.1-10 wt·ppm, and/or in that the titanium dioxideparticles have an average particle size of from 160 to 450 nm andparticularly preferably from 170 to 400 nm, and/or in that the haze liesin the range of from 0.2 to 2%.
 6. Light guide body according to one ofclaims 1 to 5, characterized in that it does not have any printingand/or any light-scattering defects.
 7. Light guide body according toone or more of the preceding claims, characterized in that the lightemitting surface is perpendicular to the light entry surface.
 8. Lightguide body according to one or more of claims 1 to 6, characterized inthat the light emitting surface is not perpendicular to the light entrysurface and/or in that the shape is a freeform body.
 9. Light guide bodyaccording to one or more of the preceding claims, characterized in thatthe light guide body comprises at least 40 wt % of polymethylmethacrylate, preferably at least 60 wt %, expressed in terms of theweight of the light guide body.
 10. Method for producing a light guidebody according to claims 1 to 9, characterized in that a light guidebody is formed from a moulding compound having at least 40 wt % ofpolymethyl methacrylate, preferably at least 60 wt %, expressed in termsof the weight of the moulding compound, and from 0.1 to 100 wt·ppm oftitanium dioxide particles, expressed in terms of the weight of themoulding compound, having an average diameter in the range of from 150to 500 nm.
 11. Method for producing a light guide body according to oneof claims 1 to 9, characterized in that an acrylic resin comprising A)0.1-100 wt·ppm of titanium dioxide particles having an average diameterin the range of from 150 to 500 nm, B) 40-99.999 wt % of methylmethacrylate, C) 0-59.999 wt % of comonomers, D) 0-59.999 wt % ofpolymers soluble in (B) or (C), the components A) to D) adding up to 100wt %, is radical-polymerized and a light guide body is thereby obtained.12. Method for producing a light guide body according to one of claims10 and 11, characterized in that the shaped body is produced by castingmethods, injection moulding, multicomponent injection moulding or byextrusion.
 13. Use of a light guide body according to one or more ofclaims 1 to 9 as transparent glazing elements, which can be used as flatlights by switching on the light sources fitted on the light entrysurfaces.
 14. Use of a light guide body according to one or more ofclaims 1 to 9 as signs, display elements, advertising objects etc.,which render particular symbols or signs visible only after switchingon.